Radiant barriers are installed in homes to reduce solar heat gain in the summer time by reducing the radiant energy allowed to pass through the attic.
When the sun heats your roof the majority of it is caused by radiant energy. This radiant heat energy is then radiated from your roof to the cooler surfaces in your attic such as your air conditioning ducting and your ceiling.
A properly installed radiant barrier reduces this radiant heat transfer making your home cooler and lowering your utility bills.
There are many different types of radiant barrier available today and not all are created equal so it's important first to know that the product you install is a quality product and second that it's installed properly.